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jQuery map vs. each作者该解法对吗？

2015-06-11 22:58 721 查看

map

and

each

functions seem to do the same thing. Are there any practical differences between the two? When would you choose to use one instead of the other?

The

each

method is meant to be an immutable iterator, where as the

map

method can be used as an iterator, but is really meant to manipulate the supplied array and return a new array.

Another important thing to note is that the

each

function returns the original array while the

map

function returns a new array. If you overuse the return value of the map function you can potentially waste a lot of memory.

For example:

var items = [1,2,3,4];

$.each(items, function() {
alert('this is ' + this);
});

var newItems = $.map(items, function(i) {
return i + 1;
});
// newItems is [2,3,4,5]
You can also use the map function to remove an item from an array. For example:

var items = [0,1,2,3,4,5,6,7,8,9];

var itemsLessThanEqualFive = $.map(items, function(i) {
// removes all items > 5
if (i > 5)
return null;
return i;
});
// itemsLessThanEqualFive = [0,1,2,3,4,5]

　　

You'll also note that the

this

is not mapped in the

map

function. You will have to supply the first parameter in the callback (eg we used

above). Ironically, the callback arguments used in the each method are the reverse of the callback arguments in the map function so be careful.

map(arr, function(elem, index) {});
// versus
each(arr, function(index, elem) {});

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jQuery map vs. each作者该解法对吗？

In jQuery, the map and each functions seem to do the same thing. Are there any practical differences between the two? When would you choose to use one instead of the other?

In jQuery, the
map
and
each
functions seem to do the same thing. Are there any practical differences between the two? When would you choose to use one instead of the other?